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Abstract The properties of optical coatings play an important role in precision experiments. Gravitational-wave detectors not only require the highest quality coatings but also ones with relatively large diameters. Coatings made with amorphous material can be scaled up easily but for now exhibit relatively high thermal noise characteristics. Crystalline coatings show great promise in terms of their thermal noise properties but cannot easily scale up to the needed sizes. In this paper we explore the possibility of a piecewise coating that includes both amorphous and crystalline material. Specifically, we estimate the scattering losses of such a piecewise coating as a function of the mismatch in the interface between the two coatings. The calculation should be taken as a lower limit to the total losses, as other surface imperfections will play an important role in the final result, but are not considered here. Finally, we present a measurement of the scattering losses from a piecewise coating, to showcase the challenges of realizing such a design. 

Optical coatings play a vital role in sensing technologies. The development of new coatings that exhibit minimal optical losses requires a detailed understanding of the development of defects within them. Current methods of defect characterization involve direct microscope imaging or x-ray diffraction studies in the case of crystallites. In this paper, we demonstrate the characterization of coating defects using light scattering, which can yield information about their size, location, and index of refraction. The method requires measuring the scattered power of each individual defect as a function of angle and comparing the data with theoretical models. Finally, we argue that this method can be used for the determination of the defect location within a multi-layer stack.